Setting a Measure
What is a Measure?
A Measure is the formula of measure calculation to determine the numerical value of the physical quantity, the notion or the function selected.
How to set a Measure?
Setting one measure to an area
Select the area to measure.
From the Measure list, select the measure to set.
Setting several measures to an area
Select the Measure cell of the area for which to add another measure.
Click Add area or measure .
From the Measure list, select the measure to set.
What are the measures available?
The topic considers photometric units and maps. If you make radiometric analysis, the units and maps correspondence are:
Unit |
Map |
|||
---|---|---|---|---|
Photometric |
Radiometric |
Photometric |
Radiometric |
|
lx |
W.m-2 |
Illuminance |
Irradiance |
|
cd |
W.sr-1 |
Intensity |
Intensity |
|
cd.m-2 |
W.sr-1.m-2 |
Luminance |
Radiance |
|
lm |
W |
The topic presents the different formula of measure calculation considering the following variables:
N = number pixels in the measurement area.
V = value of a pixel.
W = pixel weight.
Pixels on the edges of the measurement area can be partially integrated in the area, hence the formula considers the pixel weight to calculate the selected measure of the pixel part integrated in the area.
Maximum and Minimum
Maximum and Minimum correspond to the Maximum or Minimum of the selected magnitude in the measurement area defined.
These thresholds are used to exclude values in the map (below the Minimum and/or over the Maximum).
It can be useful to ignore black pixels for example. If some pixels are not lit, they introduce a bias into the measurement. A minimum or maximum threshold then allows you to ignore these values.
Average
Average corresponds to the average value of the selected magnitude in the measurement area defined.
Area
To display an area value:
Make sure the shape is a rectangle, an ellipse or a polygon.
In the Magnitude column, select Dimension.
The Measure turns into Area and its value is displayed in the Value column.
Flux or Integral
The flux or integral of the measurement area defined is measured thanks to the following formula:
F (flux) corresponds to the luminous flux in lumen (lm) for an illuminance map.
F (integral) corresponds to the luminous intensity in candela (cd) for a luminance or intensity map.
- S = surface (m²) of one pixel for a luminance or illuminance map, or a pixel solid angle in steradian (sr) for an intensity map.
Sigma
Sigma corresponds to the standard deviation of the measured magnitude.
= average of the magnitude of the measurement area defined.
Contrast
Contrast corresponds to the scale difference between the maximum and minimum magnitude of the result.
Contrast is generally used to measure the quality of an optical system.
Contrast = 0,58 |
Contrast = 1 |
RMS Contrast
RMS (Root Mean Square) Contrast corresponds to the standard deviation of the magnitude measured divided by the average value of the magnitude.
RMS Contrast can be used to measure the noise in the result.
- σ = standard deviation of the measured magnitude.
-
= average of the magnitude of the measurement area defined.
RMS Contrast = 0,39
RMS Contrast= 0,87
The higher the RMS Contrast, the lower the noise.